Casing for a writing instrument



FIG. 1 is a left side elevational view of a casing for a writing instrument showing my new design;

FIG. 2 is an enlarged scale fragmentary cross-sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a front elevational view of FIG. 1;

FIG. 4 is a bottom plan view thereof; and

FIG. 5 is a top plan view thereof. 

The ornamental design for a casing for a writing instrument, as shown and described. 